Internal combustion engines used for mobile or stationary applications are currently subject to strict emission limits. In-cylinder design changes have fallen short of allowing these engines to meet these emissions limits. Technology advances in the catalyst field have made it possible for integrated systems of engine and exhaust treatment to achieve extremely low emissions. Even so, more emission reduction efficiencies are sought from existing systems and new catalytic reduction solutions are needed to achieve even lower emissions.
In diesel engines, especially those that are classified as heavy-duty, gaseous emissions of unburned hydrocarbon and carbon monoxide are extremely low due to the lean burn nature of diffusion combustion. However, both nitric oxide and particulate matter emissions present a challenge to engineers. The challenge for catalytically treating and reducing nitric oxide is presented due to the excess oxygen in the exhaust of a typical diesel engine. The challenge of reducing particulate matter results from the nature of diffusion combustion. In spite of the recent advances in combustion technology, diesel engines are now requiring particulate filters to satisfy regulations.
Filtration of diesel particulate matter is achieved at a high efficiency rate (over 90 percent) when the exhaust is allowed to pass through porous media such as in ceramic cordierite filters (wallflow). However, a penalty associated with such high filtration efficiency is high exhaust system restriction. This exhaust restriction increases as trapping continues, and generally adversely impacts engine performance and fuel economy.
Recent work by some filter system manufacturers has resulted in filters that are characterized by “flowthrough” exhaust. As compared to wallflow filters, flowthrough filters have less exhaust restriction, but are also less efficient.